The present invention relates to a negative-type planographic printing plate having ink-repellent silicone rubber in a non-imaged area and is capable of printing in the absence of dampening.
Heretofore, several proposals have been made with reference to a dry planographic plate having an ink-repellent layer of silicone rubber, all of which, however, have various deficiencies such as having inferior strength of the printing plate or presenting problems in developing method. For example, what is disclosed in Japanese patent publication No. 2361/1972 is a presensitized planographic printing plate providing a photo-insolubilizable water soluble diazonium resin layer on the layer of silicone rubber. When a negative transparency is put on said plate and exposed to light and thereafter treated with aqueous developing liquid, the diazonium resin layer in the unexposed area dissolves to expose the surface of the silicone rubber layer to form an ink-repellent non-imaged area, whereas in the exposed area, the hardened diazonium resin layer remains to form an ink-receptive image area. However, there is insufficient adhesion between the diazonium resin layer in the image area and the underlying silicone rubber layer, and this deficiency of the planographic printing plate is such that the diazonium resin layer of the image area is peeled by inking rollers during printing or is removed by careless scratch.
An object of the present invention is to overcome such deficiencies and to provide a negative-type planographic printing plate excellent in adhesive strength of the ink-receptive image area, surface strength of the printing plate, storability, scratch resistance and high durability in long run printing.
Another object of the present invention is to provide a negative-type planographic printing plate having excellent reproducibility of image and broad permissible range of the exposing time.